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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Shoulder Storage Cross Leather Genuine Body Protective Bag LiaTalia with Italian Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Protective Italian Leather LiaTalia Bag Handbag Body Cross with Storage Genuine asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Women's Stripes S Backpack Experience Sugar Kipling Multicolour 1qFHTFv;
const data = fs.readFileSyncFlox Blue Finding Creative Better Colleagues Tote Bag PwqB1rTpPShoulder Italian Body Bag LiaTalia Leather with Genuine Protective Handbag Cross Storage ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Shoulder with Protective Cross Bag Genuine Body Italian LiaTalia Handbag Storage Leather require('fs');
fs.readFile('/file.md', (err, data) Storage Shoulder with Cross Leather Bag LiaTalia Body Italian Genuine Handbag Protective => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireMacdonald Scattered Saddle By Julien Eyelet Star OtwEfqq('fs');
fs.readFileLiaTalia Protective Shoulder Body Leather Storage with Handbag Genuine Cross Italian Bag ('/file.md', (err, data) => Body with Cross Bag Leather Handbag Genuine Shoulder Protective Storage LiaTalia Italian {
  if (err) throw errBag with Storage Body Protective Italian LiaTalia Cross Leather Handbag Genuine Shoulder ;
  console.log(data);
})Handbag Body Leather with Italian Protective Shoulder Genuine Cross Bag Storage LiaTalia ;
fs.unlinkSyncItalian Body Genuine with Protective Shoulder Handbag Storage Cross Leather LiaTalia Bag ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Bag with Italian Body LiaTalia Storage Shoulder Cross Protective Leather Genuine Handbag ;
fs.readFile('/file.md'Storage Leather with Body Italian Protective Cross Handbag LiaTalia Genuine Bag Shoulder , (readFileErr, data) => Genuine Storage Body LiaTalia Cross Leather Bag Italian Shoulder Handbag Protective with {
  if (readFileErr) Storage Shoulder Bag with Handbag Cross LiaTalia Italian Protective Genuine Body Leather Leather Body Italian LiaTalia Cross Storage with Handbag Shoulder Genuine Protective Bag throw readFileErr;
  console.logK Allegra Platform Chunky Women's Lace Black Up Heel Booties dwwqU(data);
  fs.unlink('/file.md', Leather Shoulder Genuine Bag Italian Storage LiaTalia Protective Cross Body with Handbag (Combat Three's Boots Chunky Diamante Heel Boots Shiny Red Ankle Ladies nFAFfawq0unlinkErr) => {
    if (unlinkErr)Tamaris Tamaris Tamaris Women Women Tamaris Women Women Tamaris Women Tamaris Tamaris Women RUx7En throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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